Aircraft are typically equipped with electrical air heaters that heat air supplied through an air duct to one or more exit registers in the cabin interior. The air to be heated may be drawn from outside of the aircraft, recirculated cabin air, or a mixture of outside air and cabin air, and the heater may typically raise the temperature of the air by approximately 45° F. The flow of air through the heater can be produced by either a remote, electrically powered fan, or by a fan positioned immediately adjacent the heater. Such a heater typically has heating elements that are enclosed within a protective, electrically non-conductive shell to prevent accidental contact with the heating elements. With electrical air heaters of this type, the pressure drop experience by the air passing through the heater affects the size, number, and capacity of the fans required to circulate the air at a desired rate. It is therefore desirable to minimize this pressure drop. Further, it is desirable to heat the air passing through the heater evenly to eliminate the need for additional air mixing devices. Such devices complicate the air heating system, and produce an additional drop in the pressure of the air, requiring even larger and more powerful fans. An aircraft air heater must also be able to withstand sudden changes in the flow rate of the air without damage, and without adversely affecting the temperature of the air leaving the heater. All aircraft devices, including air heaters, must past a battery of certification tests, demonstrating the ability to withstand exposure to severe vibration, pressure variations, temperature variations, and other tests, as well. A further, significant design goal of any aircraft component, including air heaters, is that the component be as light as possible.
Aircraft air heaters have, in the past, incorporated a number of thin, flat electrical heating elements, producing heating with reduced heat stratification. Typically, thin, light heating elements, each consisting of a resistance heating layer, surrounded by layers of fiber reinforced composite, have been used to provide good heat transfer. However, unless the flat heating elements are reinforced with additional materials, or supported with secondary internal structures that adversely reduce the airflow area, these flat components have been found to be too weak to withstand the vibration to which aircraft heaters are subjected in normal operation. Incorporating support structures into the interior of such a heater, however, has resulted in restricting the flow of air through the heater, causing a greater pressure drop and more uneven heating. An additional drawback of using thin, flat heating elements is that they may produce an air flow pattern that switches sporadically between laminar and turbulent flow under minor variations in air flow rate. This change between laminar and turbulent flow reduces heater efficiency, and can also cause damage to the heater due to overheating of portions of the heater elements.
Finally, it is desirable that an air heating assembly be electrically isolated, with no conductive electrical path outside of the heating elements themselves. This simplifies heater installation in all-composite aircraft, and saves additional weight by eliminating all grounding straps.